North Korea faces consequences if it goes ahead with a second nuclear test, but little can be done to stop it, the U.S. special envoy to the communist state said on Friday.

Stephen Bosworth said after meeting officials in Seoul that the United States was leaving the door open for dialogue, despite the fact that Pyongyang said it was useless to talk to the Obama administration because its “hostile policy” left it no choice but to bolster its nuclear deterrent.

“If the North Koreans decide to carry out a second nuclear test, we will deal with the consequences of that. And there will be consequences,” Bosworth told reporters after meeting South Korea’s foreign minister.

“But we can’t control at this stage what North Korea does.”

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A South Korean official familiar with the North said there was increased activity at North Korea’s known nuclear test site, suggesting Pyongyang was gearing up for a new test.

Experts said it could take a few weeks for the North, which conducted its first nuclear test in October 2006, to prepare for another one.